How would I check if an object is in the scene?

Hello. I have some enemies which are instantiated is random positions at random times of the map. The more you shoot, the more gets instantiated at that present time. When the player doesn't kill an enemy and all the enemies are destroyed through time. Nothing happens. How would I go about, checking if an enemy is in the scene and if not, loadlevel("highscore_board"). I know about the if not business. I just need an api to see if a enemy is in the scene.